1. Field of the Invention
The present invention relates to a system for measuring a voltage of each of cells constituting a battery assembly having a large number of cells, which are connected in series.
2. Description of the Related Art
In hybrid vehicles, electric vehicles, fuel-cell vehicles or the like, it has become common to configure a high-voltage battery assembly by connecting in series a plurality of unit batteries including rechargeable batteries or fuel cells in order to reduce the loss due to wiring resistance or reduce the size of a switching element. In the case that a fuel cell is used, for example, a cell voltage of a fuel cell is around 1 volt, and therefore generally several hundreds of cells are connected in series to provide a required high voltage. In such a case, when one of unit batteries has a trouble and operates with an extreme low or high voltage, the unit battery having a trouble can cause corrosion or insufficient voltage resistance, which can result in the breakage of a whole battery assembly. Therefore, when such a battery assembly is used, a voltage of one unit battery or voltages of a plurality of unit batteries are sequentially scanned at a time to measure and monitor a voltage, thereby making it possible to promptly deal with any trouble.
There have been known examples for monitoring a voltage as mentioned above. Japanese Laid-open Patent Application Hei 11-113182 (Paragraph [0017]; FIG. 1) discloses an example for detecting a voltage of each of unit batteries by different differential amplifiers by using a bottom terminal as a reference. Japanese Laid-open Patent Application No. 2003-70171 (Paragraph [0017]; FIG. 1) discloses an example for detecting a voltage of each of batteries by using a virtual ground, which has the same electric potential as that of a midpoint of an battery assembly, as a reference.
Japanese Laid-open Patent Application No. 2002-156392 (Abstract; FIG. 1) discloses a so-called flying capacitor technique. In this technique, a voltage of each unit battery is sequentially applied to a flying capacitor through a pair of multiplexers, and then the multiplexers are shut off to sample and hold the voltage of the unit battery. Then, each end of the flying capacitor is electrically conducted to a voltage detection circuit via an analog switch, which is designed to output an electrical potential of a capacitor, so that a potential difference of the flying capacitor, i.e. a storage voltage of the flying capacitor is detected by the voltage detection circuit.
In an example disclosed in Japanese Laid-open Patent Application No. 2002-122643 (Paragraph [0044-0048]; FIG. 1), a voltage of each unit battery is input to a pair of input terminals of a first stage differential amplifier via a pair of multiplexers. Then, the first stage differential amplifier differentially amplifies a voltage on a reference potential of a bottom potential of a battery assembly. An output voltage of the first stage differential amplifier is amplified by a subsequent stage differential amplifier by using a ground potential of a vehicle body as a reference, and then an output voltage of the subsequent stage differential amplifier is A/D converted.
In the above-mentioned systems in Japanese Laid-open Patent Applications Hei. 11-113182 and No. 2003-70171, however, a reference voltage of a first stage differential amplifier or a converter, which detects an input voltage of a unit battery, is a bottom terminal or a virtual ground having the same potential as that of a midpoint of a battery assembly. In such a case, a voltage to be input to a first stage amplifier or a converter increases. Therefore, it is difficult to obtain a large gain, and the accuracy of the voltage detection is deteriorated. Furthermore, an increase of a dark current causes a problem that the accuracy of the voltage detection can not be sufficiently obtained. In the flying capacitor technique as disclosed in Japanese Laid-open Patent Application No. 2002-156392, a high voltage section on the battery side and a low voltage section connected to a ground potential of a vehicle body are electrically insulated from each other so that elements need to be provided with voltage resistance in both directions. Therefore, a large number of elements having high voltage resistance in both directions are necessary, which results in the increase of the cost. Furthermore, in above-mentioned Japanese Laid-open Patent Applications Nos. 2002-156392 and 2002-122643, a switch is changed over to input a voltage in a common microcomputer and a common A/D converter. However, such configuration is likely to be affected by noises due to parasitic capacity or the like. With the configuration disclosed in Japanese Laid-open Patent Application No. 2002-122643 wherein a voltage is measured by using a grounded vehicle body as a reference, a reference potential of a battery assembly and a potential of a vehicle body are electrically insulated from each other. Therefore, there easily occurs a noise, a cause of which is difficult to determine, and the configuration is likely to be affected by parasitic noises.